The present invention relates to a magnetoresistive element.
In recent years, massive efforts have been made to develop elements utilizing spin polarization of conduction electrons in a ferromagnetic material. A typical example of such devices is a spin valve type magnetoresistive element (Phys. Rev. B, 43 (1991) 1297). A spin valve type magnetoresistive element has a structure obtained by stacking first and second ferromagnetic layers (such as NiFe and Co), each having a thickness of about 5 to 10 nm, with an intervening non-magnetic layer (such as Cu) having a thickness of about 2 to 5 nm between them and further stacking an antiferromagnetic layer (such as FeMn) on one of the ferromagnetic layers. In this structure, the magnetization direction of the ferromagnetic layer in contact with the antiferromagnetic layer is fixed and only that of the ferromagnetic layer not in contact with the FeMn layer changes according to a signal magnetic field. The element shows a high resistance when the magnetization directions of the upper and lower ferromagnetic layers are in anti-parallel alignment due to spin-dependent scattering and shows a low resistance when they are in parallel alignment because no such scattering occurs. Thus, the resistance of the element can vary remarkably vary between the two states.
However, the change in the resistance of a spin valve type magnetoresistive element does not greatly exceed 10%. Additionally, since each of the layers of the spin valve type magnetoresistive element are made of metal, its specific resistance is low and hence a large current density is required to obtain a sufficiently large signal output from the element. This means that the heat generated in the element becomes a problem when the element is miniaturized to integrate the elements at a high density.
Recently, there has been proposed an element called a spin transistor. The spin transistor basically has a three-layered structure of ferromagnetic layer/non-magnetic layer/ferromagnetic layer and operates in a manner as described below. Spin-polarized electrons are injected into the non-magnetic layer (for example, Cu) from one of the ferromagnetic layers to bring the non-magnetic layer into a non-equilibrium state, which is detected by observing the potential difference between the other ferromagnetic layer and the non-magnetic layer. Under this condition, a positive potential is observed when the magnetization directions of the two ferromagnetic layers are in parallel alignment, whereas a negative potential is observed when they are in anti-parallel alignment. Since the relative magnetization directions of the two ferromagnetic layers can be transformed into a potential difference, such an element can be used as a magnetoresistive element or a transistor.
However, since all the layers of a spin transistor are made of metal, it can produce only an output voltage of about 1 mV when it has a size of 1 .mu.m.sup.2. Additionally, such a transistor barely provides an amplifying effect.
In view of the above described circumstances, the inventors of the present invention proposed a magnetoresistive element comprising a pnp transistor and first and second ferromagnetic layers formed in contact with an emitter layer and a base layer, respectively, and designed to utilize the spin-dependent recombination of carriers.
With such an element, the recombination time is required to be shorter than the spin relaxation time in order to utilize the spin-dependent recombination. Hence, it is accompanied by a problem that it cannot provide a large rate of output change if an indirect transition semiconductor such as Si is used. There is a need to improve the effect of such an element by minimizing the above identified problem.